In the third generation partnership project (3GPP) a proposal called D-TxAA is under discussion for UMTS as a way of increasing the peak bit rate. This is derived from an existing closed loop transmit diversity scheme (TxAA mode 1) where the mobile terminal signals to the network complex weights which should be applied to the signals from each of two transmitting antennas. In D-TxAA two different data streams are transmitted using orthogonal weight vectors creating two different beams, wherein a first weight vector for generating a first beam is typically based on the weight vectors indicated by the mobile terminal, and a second weight vector for generating a second beam is derived deterministically from the first vector. The weight vector used for the transmission of one of the beams is indicated to the mobile terminal by means of signaling carried on a signaling channel known as the High-Speed Shared Control Channel (HS-SCCH). The beam for which the weight vector is signaled on the HS-SCCH is hereinafter denoted the “primary beam”, and the other beam is hereinafter denoted the “secondary beam”.
Different data blocks, known as transport blocks, are transmitted to the mobile terminal on each beam using possibly different amounts of transmission resource, such as the number of channelization codes, and possibly different transmission parameters, such as the modulation scheme. Different code rates may be employed in order to map the number of bits in each transport block onto the number of physical channel bits that may be transmitted using the selected number of channelization codes and modulation scheme. In order to enable the mobile terminal to receive and decode the transmissions, the transport block size, number of channelization codes and modulation scheme for each beam are signaled to the mobile terminal using the HS-SCCH.
In earlier versions of UMTS, only a single data stream was transmitted to the mobile terminal, requiring only one set of signaling. When a second stream is transmitted, the corresponding signaling information causes an increase in signaling overhead, which detracts from the overall data transmission capacity of the system.
Some known methods of reducing the signaling overhead related to the transmission of a second stream include restricting the set of transport block sizes which may be transmitted on one stream relative to the transport block size that is transmitted on the other beam. However, this reduces flexibility for transmitting data on each stream at the rates required by the user applications. The restriction of the set of transport block sizes may for example be applied by restricting the difference between the numbers of channelization codes used on the two beams.
One known method for reducing the signaling overhead is to require that the identity of the first code used on each beam is the same. The other codes used on each beam then form a contiguous set starting from the first code, for example from an Orthogonal Variable Spreading Factor (OVSF) code tree.
It has also been suggested that if the number of codes allocated to primary and secondary beams are restricted to be equal (i.e. “symmetric code allocation”) then this will reduce signaling overhead (compared to full flexibility which would in UMTS need 4 bits to indicate the number of codes on the secondary beam and 1 bit to indicate the modulation scheme). However, this also implies a restriction on the range of transport block sizes which can be transmitted on one beam relative to the other. More specifically this means that there may be a problem with sending a small transport block on the secondary beam when a large one is sent on the primary beam. There is a ratio of about 3 between the maximum and minimum Transport Block Sizes which can be signaled for a given number of codes. This limitation can be partly alleviated by using a lower-order modulation scheme on the secondary beam compared to that used on the primary beam: for example, when the primary beam uses 16QAM, QPSK may be used on the secondary beam to enable a smaller transport block to be carried on the secondary beam.